Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of a person""s arterial blood, an indicator of their oxygen supply. Oxygen saturation monitoring is crucial in critical care and surgical applications, where an insufficient blood supply can quickly lead to injury or death. FIG. 1 illustrates a conventional pulse oximetry system 100, which has a sensor 110 and a monitor 150. The sensor 110, which can be attached to an adult""s finger or an infant""s foot, has both red and infrared LEDs 112 and a photodiode detector 114. For a finger, the sensor is configured so that the LEDs 112 project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode 114 is positioned at the finger tip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled xe2x80x9cLow Noise Optical Probe,xe2x80x9d which is assigned to the assignee of the present invention and incorporated by reference herein.
Also shown in FIG. 1, the monitor 150 has LED drivers 152, a signal conditioning and digitization front-end 154, a signal processor 156, a display driver 158 and a display 159. The LED drivers 152 alternately activate the red and IR LEDs 112 and the front-end 154 conditions and digitizes the resulting current generated by the photodiode 114, which is proportional to the intensity of the detected light. The signal processor 156 inputs the conditioned photodiode signal and determines oxygen saturation based on the differential absorption by arterial blood of the two wavelengths emitted by the LEDs 112. Specifically, a ratio of detected red and infrared intensities is calculated by the signal processor 156, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The display driver 158 and associated display 159 indicate a patient""s oxygen saturation, heart rate and plethysmographic waveform.
Increasingly, pulse oximeters are being utilized in portable, battery-operated applications. For example, a pulse oximeter may be attached to a patient during emergency transport and remain with the patient as they are moved between hospital wards. Further, pulse oximeters are often implemented as plug-in modules for multiparameter patient monitors having a restricted power budget. These applications and others create an increasing demand for lower power and higher performance pulse oximeters. A conventional approach for reducing power consumption in portable electronics, typically utilized by devices such as calculators and notebook computers, is to have a xe2x80x9csleep modexe2x80x9d where the circuitry is powered-down when the devices are idle.
FIG. 2 illustrates a sleep-mode pulse oximeter 200 utilizing conventional sleep-mode power reduction. The pulse oximeter 200 has a pulse oximeter processor 210 and a power control 220. The power control 220 monitors the pulse oximeter output parameters 212, such as oxygen saturation and pulse rate, and controls the processor power 214 according to measured activity. For example, if there is no significant change in the oxygen saturation value over a certain time period, the power control 220 will power down the processor 210, except perhaps for a portion of memory. The power control 220 may have a timer that triggers the processor 210 to periodically sample the oxygen saturation value, and the power control 220 determines if any changes in this parameter are occurring. If not, the power control 220 will leave the processor 210 in sleep mode.
There are a number of disadvantages to applying consumer electronic sleep mode techniques to pulse oximetry. By definition, the pulse oximeter is not functioning during sleep mode. Unlike consumer electronics, pulse oximetry cannot afford to miss events, such as patient oxygen desaturation. Further, there is a trade-off between shorter but more frequent sleep periods to avoid a missed event and the increased processing overhead to power-up after each sleep period. Also, sleep mode techniques rely only on the output parameters to determine whether the pulse oximeter should be active or in sleep mode. Finally, the caregiver is given no indication of when the pulse oximeter outputs were last updated.
One aspect of a low power pulse oximeter is a sensor interface adapted to drive a pulse oximetry sensor and receive a corresponding input signal. A processor derives a physiological measurement corresponding to the input signal, and a display driver communicates the measurement to a display. A controller generates a sampling control output to at least one of said sensor interface and said processor so as to reduce the average power consumption of the pulse oximeter consistent with a predetermined power target.
In one embodiment, a calculator derives a signal status output responsive to the input signal. The signal status output is communicated to the controller to override the sampling control output. The signal status output may indicate the occurrence of a low signal quality or the occurrence of a physiological event. In another embodiment, the sensor interface has an emitter driver adapted to provide a current output to an emitter portion of the sensor. Here, the sampling control output determines a duty cycle of the current output. In a particular embodiment, the duty cycle may be in the range of about 3.125% to about 25%.
In another embodiment, the sensor interface has a front-end adapted to receive the input signal from a detector portion of the sensor and to provide a corresponding digitized signal. Here, the sampling control output determines a powered-down period of the front-end. A confidence indicator responsive to a duration of the powered-down period may be provided and displayed.
In yet another embodiment, the pulse oximeter comprises a plurality of data blocks responsive to the input signal, wherein the sampling control output determines a time shift of successive ones of the data blocks. The time shift may vary in the range of about 1.2 seconds to about 4.8 seconds.
An aspect of a low power pulse oximetry method comprises the steps of setting a power target and receiving an input signal from a pulse oximetry sensor. Further steps include calculating signal status related to the input signal, calculating power status related to the power target, and sampling based upon the result of the calculating signal status and the calculating power status steps.
In one embodiment, the calculating signal status step comprises the substeps of receiving a signal statistic related to the input signal, receiving a physiological measurement related to the input signal, determining a low signal quality condition from the signal statistic, determining an event occurrence from the physiological measurement, and indicating an override based upon the low signal quality condition or the event occurrence. The calculating power status step may comprise the substeps of estimating an average power consumption for at least a portion of the pulse oximeter, and indicating an above power target condition when the average power consumption is above the power target. The sampling step may comprise the substep of increasing sampling as the result of the override. The sampling step may also comprise the substep of decreasing sampling as the result of the above power target condition, except during the override.
Another aspect of a low power pulse oximetry method comprises the steps of detecting an override related to a measure of signal quality or a physiological measurement event, increasing the pulse oximeter power to a higher power level when the override exists, and reducing the pulse oximeter power to a lower power level when the override does not exist. The method may comprise the further steps of predetermining a target power level for a pulse oximeter and cycling between the lower power level and the higher power level so that an average pulse oximeter power is consistent with the target power level.
In one embodiment, the reducing step comprises the substep of decreasing the duty cycle of an emitter driver output to the sensor. In another embodiment, the reducing step comprises the substep of powering-down a detector front-end. A further step may comprise displaying a confidence indicator related to the duration of the powering-down substep. In yet another embodiment, the reducing step comprises the substep of increasing the time-shift of post-processor data blocks.
Another aspect of a low power pulse oximeter comprises a sensor interface adapted to receive an input signal from a sensor, a signal processor configured to communicate with the sensor interface and to generate an internal parameter responsive to the input signal, and a sampling controller responsive to the internal parameter so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The signal processor may be configured to generate an output parameter and the sampling controller may be responsive to a combination of the internal and output parameters so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The internal parameter may be indicative of the quality of the input signal. The output parameter may be indicative of oxygen saturation.
In another embodiment, the sampling controller is responsive to a predetermined power target in combination with the internal parameter so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The signal processor may be configured to generate an output parameter and the sampling controller may be responsive to a combination of the internal and output parameters and the power target so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The sensor interface may comprise an emitter driver and the sampling control may modify a duty cycle of the emitter driver. The sensor interface may comprise a detector front-end and the sampling control may intermittently power-down the detector front-end. The processor may generate a plurality of data blocks corresponding to the input signal, where each of the data blocks have a time shift from a preceding one of the data blocks, and where the sampling control may determine the amount of the time shift.
A further aspect of a low power pulse oximeter comprises an interface means for communicating with a sensor, a processor means for generating an internal parameter and an output parameter, and a controller means for selectively reducing the power consumption of at least one of the interface means and the processor means based upon the parameters. In one embodiment, the interface means comprises a driver means for determining the duty cycle of emitter current to the sensor, the driver means being responsive to the controller means. In another embodiment, the interface means comprises a detector front-end means for receiving an input signal from the sensor, the power for the detector front-end means being responsive to the controller means. In yet another embodiment, the processor means comprises a post-processor means for determining a time shift between data blocks, the post-processor means being responsive to the controller means. In a further embodiment, the controller means comprises a signal status calculator means for generating an indication of a low signal quality or a physiological event based upon at least one of an internal signal statistic and an output physiological measurement, and a control engine means in communications with the signal status calculator means for generating a sampling control responsive to the indication. In yet a further embodiment, the controller means comprises a power status calculator means for generating a power indication of power consumption relative to a power target, and a control engine means in communications with the power status calculator means for generating a sampling control responsive to the power indication.